1. Field of the Invention
The present invention relates to a lens driving device for an optical pickup unit, and more specifically to a device having driving and supporting structure of an objective lens to be used in a pickup unit of a system for optically recording/reading information.
2. Description of Background Information
In systems for optically reading recorded information presently in wide use, a laser beam for reading information is focused on a recording track formed on the surface of a recording medium such as a video and/or audio disk by means of an optical pickup unit. The recorded information is recovered from the recording medium in terms of the change in the reflection of the irradiated reading laser beam. Due to the necessity that the reading laser beam is correctly focused on a recording surface of the recording medium irrespectively of fluctuation of the position of the recording surface, for instance caused by a curvature of the recording medium, the pickup unit is designed to include a support structure of an objective lens which allows the movement of the objective lens in a direction perpendicular to the recording surface within a relatively small amplitude.
Further, it is also necessary that the support structure allows lateral movement of the objective lens in a direction perpendicular to the direction of the recording track so that the reading laser beam always correctly follows the recording track irrespective of eccentricity of the recording track which may sometimes be present.
In order to meet such requirements, a lens driving device of an optical unit presently in use is constructed such that the so called focussing coil and tracking coils are fixed to the objective lens and commonly placed in magnetic circuits. However, in this type of conventional structure, there was a drawback that the size of the focussing coil necessarily becomes large due to the reason that a pole piece of the magnetic circuit having a through hole for permitting the reading laser beam is inserted into the focussing coil. This has been resulting in further disadvantages that the response characteristic or follow characteristic of a focus servo system as well as a tracking servo system provided to the information reading system is negatively affected. Further, it was unavoidable that the total size of the information reading system becomes large.